Dental decay and caries usually occurs as the result of attack by acids in the mouth and saliva. The hydrogen ions of such acids leach out and replace the calcium ions from the surface of the enamel. A lesion or cavity in the enamel thus occurs and if deep enough, renders the tooth vulnerable to attack by decay producing bacteria.
In an effort to protect the enamel surface from attack by such acids, soluble fluoride ions have been topically applied directly onto the surface of the enamel. The consumption of soluble fluoride compounds in water has been shown to be effective in preventing dental decay and caries, but when applied topically to tooth enamel, fluoride ions do not contribute to any significant reduction in dental decay or caries because the fluoride is readily washed off the tooth enamel.